The present invention relates to an exhaust gas purification device for an internal-combustion engine, and more particularly it relates to a control scheme for burning and removing suspended particle matters (SPM, hereinafter referred to as a “particulate”) which are generated from an internal-combustion engine and trapped by a particulate filter that is operable to trap and remove the particulates.
Particulates which are exhausted from an internal-combustion engine includes such particle matters as soot particles, various hydrocarbon particles and other insoluble organic matters (SOOT) on which unburned fuel, unburned oil constituents and other soluble organic matters (SOF) and/or sulfur oxide matters are deposited, the sulfur oxide matters being produced. There has been conventionally developed a diesel particulate filter (DPF) for trapping those particulates contained in the exhaust gas. Through use of such particulate filter, it is possible to trap more than 95% of the particulates contained in the exhaust gas.
A particulate filter comprises porous honeycomb cores of ceramic or the like so as to trap the particulates when the exhaust gas passes through the wall of porous material. Accordingly, as the particulates are deposited, the exhaust gas pressure increases. The increase in the exhaust gas pressure may cause a backpressure to the internal-combustion engine, which may further result in deterioration of fuel consumption and/or bad condition of the internal-combustion engine. Therefore, it is required to remove the particulates trapped by the particulate filter to regenerate or clean the filter every time the travel distance of the vehicle reaches several hundred or several thousand kilometers.
As an example of a system for regenerating or cleaning a particulate filter of the vehicle, the Japanese Patent Application Unexamined Publication (Kokai) No. HEISEI 8-338229 discloses an exhaust gas purification device comprising an electric heater, an oxidation catalyst and a particulate filter which are arranged within an exhaust gas passage in this sequence from the upstream side wherein at a time of a filter cleaning, an intake air amount is reduced and diesel fuel is supplied to an exhaust pipe while the electric heater is heated.
However, when the fuel is supplied to the exhaust pipe, reduction of the intake air amount in order to raise the exhaust gas temperature produces a problem that the emission gets worse due to deterioration of the combustion condition of the internal-combustion engine.
Besides, as for diesel engines, lean burn operations with excessive air are conventionally carried out in almost all operational ranges. No sensors that can detect the air/fuel ratio in the lean range with higher precision have been developed. Thus, there are no systems for performing a feedback control of the air/fuel ratio of the exhaust gas. For this reason, there are such problems as the emission gets worse due to instability of the air/fuel ratio of the exhaust gas, and as the regeneration process for the particulate filter cannot be stabled due to instability of the exhaust gas temperature when the particulate filter is cleaned through post-injection.
Thus, it is an objective of the present invention to provide an exhaust gas purification device for an internal-combustion engine which can solve at least one of the above-described problems.